Over time, amalgam fillings tend to distort, corrode, and change shape. This causes the tooth to become grey as the metal rusts: leaching into the tooth. During this time, the amalgam will expand and cause fracture lines, that if go untreated, will cause the tooth to break. Why wait for these fillings to crack your tooth? Changing them out before they cause this type of damage will be less costly to saving the tooth in the future. A simple filling change costs a few hundred dollars, compared to the cost of a crown(~$1100) and even worse – an implant(~$5000) should tooth loss occur.